This application relates to and incorporates by reference Japanese patent application no. 2001-353606, which was filed on Nov. 19, 2001.
The present invention relates to a capacitive humidity sensor having a pair of comb-shaped electrodes on a surface of a substrate.
In a conventional capacitive humidity sensor, a lower electrode is formed on a substrate. A humidity-sensitive film, made of a polyimide material, the capacitance of which changes with humidity, is formed on the lower electrode. A thin, upper electrode, through which humidity can penetrate, is formed on top of the humidity-sensitive film. This type of device is called a vertically integrated sensor.
In this type of device, the lower electrode is formed on the substrate using a semiconductor process. Then the work piece is taken off of the semiconductor manufacturing line to deposit an organic humidity-sensitive film. Then the work piece is placed back on the semiconductor production line to form the top electrode. For this reason, the semiconductor production line is exposed to a risk of equipment contamination, requiring a special facility for forming the upper electrode. Therefore, the existing semiconductor production line cannot be used as is.
To address this issue, the inventors of the present invention have developed a capacitive humidity sensor, which is shown in FIG. 5. In this device, a pair of comb-shaped electrodes 31, 32 face each other and lie in the same plane, but are isolated from each other, on a surface of a substrate 10, so that the teeth of the comb-shaped electrodes are interdigitated. A humidity-sensitive film 50 is formed over the pair of comb-shaped electrodes 31, 32 and areas between the teeth of the comb-shaped electrodes that face each other. Humidity is detected based on a capacitance value between the pair of comb-shaped electrodes 31, 32, which changes in accordance with humidity changes in the atmosphere.
Such a device can be manufactured by forming the humidity-sensitive film 50 after the pair of comb-shaped electrodes 31, 32 are formed on a surface of the substrate 10, using a semiconductor process. Therefore, this device can be easily manufactured using an existing semiconductor production line.
The vertically integrated sensor mentioned earlier, however, makes effective use of the surface of the substrate, where the electrodes are formed. The sensor shown in FIG. 5, on the other hand, relies on surfaces at the edges of the electrode films.
For this reason, the distance along which the electrodes face each other in the sensor shown in FIG. 5 is smaller than in the vertically integrated sensor, when substrates of comparable sizes are used. As a result, the magnitudes of changes in the capacitance are smaller in the sensor shown in FIG. 5. In order to ensure changes in the capacitance sufficient for sensor output, a larger substrate surface would be required.
To address the issue described above, the objective of the present invention is to efficiently achieve relatively large capacitance changes in spite of a small substrate surface area.
The inventors have discovered that the capacitance changes by different magnitudes at different tooth widths of the comb-shaped electrodes. Also, the capacitance varies according to the spacing between the facing electrodes, or according to the spacing between the comb teeth, in the capacitive humidity sensor of FIG. 5.
The inventors performed experiments for achieving large capacitance changes, when the electrodes occupy a relatively small surface area, by optimizing the width of the electrodes and the spacing between the electrodes. The present invention has been made based on these experiments.
The present invention is a capacitive humidity sensor that includes a substrate; a pair of electrodes, which face each other on a surface of the substrate on the same plane but in isolation from each other; and a humidity-sensitive film, which covers the electrodes and an area between the electrodes. The comb-shaped electrodes have interdigitated teeth. Humidity is detected based on the capacitance between the pair of electrodes, which changes with changes according to the humidity in the atmosphere. The width of each tooth in the pair of electrodes is L1, and the distance between a tooth of one of the electrodes and a tooth of the other electrode, which face each other, is L2. When the width L1 is less than 3 xcexcm, the distance L2 is 5 xcexcm. When the width L1 is greater than or equal to 3 xcexcm, the distance L2 is less than or equal to 5 xcexcm.